The present invention relates to hybrid devices of the pneumatic and positive-displacement hydraulic actuators generally named xe2x80x9chydropneumatic actuatorxe2x80x9d. A hydropneumatic actuator per the present invention has a broad spectrum of applications in many industrial fields, and can be used for actuating a variety of machine parts and objects. More particularly, this invention relates to improvements in pneumatic actuators utilizing positive-displacement zero volumetric differential hydraulic damping means for achieving smooth displacement, rapid stopping and steady, and accurate positioning of the actuator.
Pneumatic actuators (piston-cylinders, rotary actuators, etc.) are generally advantageous in respect to low purchase and operation cost over positive-displacement hydraulic actuators. The simplicity of using one centralized station producing compressed air (which in some instances is capable of supplying a whole plant with air power), cheap of-the-shelf pneumatic hardware and means of control (such as hoses, fittings, switches, valves, etc.) makes pneumatics almost a plug-in technology.
Pneumatic actuators, however, have certain disadvantages. For example, they suffer rapid accelerations (which normally happens at the beginning of actuation) and xe2x80x9ccreepingxe2x80x9d (when the compressed air is cut off, but the actuator keeps moving). These effects are attributed to the compressibility of air. Using pneumatic actuators it is very difficult to achieve accurate control of speed and displacement, or maintain a steady position of an actuator. In fact, achieving the quality of motion and position control equivalent or even any close to the quality of motion and position control routinely achievable by positive-displacement hydraulic systems is practically unrealistic.
Positive-displacement hydraulic actuators, on the other hand, offer an excellent motion and position control, but the cost of hydraulic systems as well as the maintenance of hydraulics is high. In addition, most hydraulic systems require individual pump stations, which makes them even more expensive and further complicates the their usage.
The present invention offers an inexpensive hybrid actuator that allows to combine the advantages of the pneumatic and positive-displacement hydraulic actuation. The present invention gives a viable alternative to those areas of the industry where the need in accurate control of motion and position is contradicted by a low cost requirement.
It is known in the art to utilize positive-displacement hydraulic actuators in combination with pneumatic actuators. In such hybrids a displacement that takes place in a pneumatic actuator is being translated into a displacement of a positive-displacement hydraulic actuator filled with damping fluid, thus causing a flow of dampening fluid in the hydraulic actuator. The accurate control of motion and position is then achieved through controlling the flow of dampening fluid using a variety of optional valve means and their combinations.
U.S. Pat. No. 2,624,318 to B. Walder, et. al. shows a pneumatic cylinder with a hollow piston rod serving as a housing for a hydraulic actuator containing dampening fluid which travels from one side of the hydraulic actuator plunger to the other.
This invention uses a single rod hydraulic actuator for damping the pneumatic cylinder. The obvious disadvantage of such an arrangement is the presence of a volumetric differential in the damping cylinder (that is natural for single rod hydraulic actuators). To compensate for the volumetric differential of the damping hydraulic actuator the device is equipped with an additional expendable reservoir for receiving, containing and returning back to the system differential volumes of damping fluid.
U.S. Pat. No. 3,146,680 to James F. Hutter, et. al. shows a hydraulically controlled pneumatic cylinder with a hollow piston rod utilized as the housing of a single rod hydraulic actuator. The hollow piston rod of the pneumatic cylinder is filled with oil. The two chambers of the hydraulic actuator are connected through an oil reservoir with a floating cover and a valve means that allow to control the oil flow between the two chambers of the cylinder.
Similar to the first prior art described, this invention uses a single rod hydraulic actuator (with a natural volumetric differential), and an expandable oil reservoir to compensate for the volumetric differential of the hydraulic actuator.
The expandable reservoirs used in both cases are in essence a form of a hydraulic accumulator means and, thus, are equipped with some type of a built-in spring (mechanical, pneumatic, etc.) that makes them expandable. At the same time, the built-in spring reintroduces the main disadvantage of a true pneumatic actuatorxe2x80x94compressibility of the media. Therefore, the utilization of expandable reservoirs defeats the very object or minimizes the extent of improvement attempted by the prior arts described above.
In addition, the complex switches and valve means utilized to control the fluid transfer between the chambers of the hydraulic actuator and through the expandable reservoirs complicate such hybrid actuators, making them more expensive, and less reliable.
U.S. Pat. No. 3,313,214 to Nathan Ackerman shows a hydropneumatic feedxe2x80x94a hydrid of pneumatic and single rod hydraulic cylinders. This hydropneumatic feed also includes a spring-loaded fluid reservoir of an expandable nature so to compensate for the volumetric differential of the single rod hydraulic cylinder which is built into a piston rod of the pneumatic cylinder. Therefore, this hydrid shall suffer the same disadvantages as the prior arts discussed above.
U.S. Pat. No. 3,678,805 to Henry Walter Weyman shows a pneumatic cylinder assembly incorporated with single rod hydraulic damping. In this invention a built-in spring-loaded fluid reservoir of an expandable nature is also used to compensate for the volumetric differential of the single rod damping hydraulic cylinder.
U.S. Pat. No. 5,735,187 to Bert Harju shows a pneumatic cylinder with an integrated hydraulic control system and a single rod hydraulic damping cylinder. The arrangement of this invention does not show any special means to compensate for the volumetric differential natural to a single rod hydraulic cylinder. Thus, in order for the hybrid cylinder to be functional the single rod hydraulic actuator shall be partially filled with damping fluid. In fact, the total volume of the damping hydraulic fluid shall be no greater than the full volume of the small chamber of the single rod hydraulic damping cylinder. Therefore, the larger chamber of the hydraulic actuator per this invention will develop a vacuum gauge pressure at all positions of the plunger except the terminal position at which the plunger is fully retracted. Due to the presence of a vacuum gauge pressure in one of the chambers the arrangement of this invention will suffer the same disadvantage of media compressibility as all the prior arts discussed above.
The concept of a hybrid of positive-displacement hydraulic and pneumatic actuators was practically utilized in commercially available devices named xe2x80x9cCyl-Checkxe2x80x9d by Allenair Corporation. The xe2x80x9cCyl-Checkxe2x80x9d design arrangement, however, uses single rod hydraulic damping cylinders and spring-loaded fluid reservoirs as well, to compensate for a volumetric differential of the single rod damping hydraulic actuators.
Whatever the precise merits, features and advantages of the above cited references, all of them suffer the same main disadvantage attributed to the use of damping hydraulic actuators with positive volumetric differential. Thus, none of them achieve or fulfill the goal of providing an inexpensive technology which combines the advantages separately inherent to pneumatic and positive-displacement hydraulic actuation.
It is therefore, a principle object of the present invention to provide a hydropneumatic actuator capable of smooth actuation which speed and positioning can be controlled with high level of accuracy.
Another object of the present invention is to provide a free of xe2x80x9ccreepingxe2x80x9d and rapid speed changes hydropneumatic actuator powered by compressed gasses and yet.
It is also an object of the present invention to provide an inexpensive and reliable hydropneumatic actuator.
Yet another object of the present invention is to provide a hydropneumatic actuator capable of rapid and accurate stops in any required position.
The present invention achieves the forgoing objectives by the use of pneumatic actuators combined with a positive-displacement hydraulic damping means with zero volumetric differential (such as double rod hydraulic actuators with constant diameter of the rod on both sides of the piston, bellows with equal volumetric to linear displacement ratios, etc.) which allows dampening fluid transfer between its chambers without producing vacuum as well as excessive amounts of dampening fluid (that would require additional spring-loaded fluid reservoirs of an expandable nature).
Such hydropneumatic actuators are simple by design, and inexpensive due to the small number of components from which they can be constructed. The majority the components can be mass produced or off-the-shelf items.